1. Field of the Invention
The prevent invention relates to a photoelectric conversion device and a method of producing the device, and more particularly, to a photoelectric conversion device comprising at least an electron acceptive charge transfer layer, an electron donative charge transfer layer, and a light absorption layer formed between these charge transfer layers and a method of producing the device.
2. Related Background Art
A solar cell utilizing a semiconductor junction of silicon, gallium arsenide or the like is generally known as a method of converting light energy into electric energy. A crystal silicon solar cell and a polycrystalline silicon solar cell utilizing a p-n junction of a semiconductor, and an amorphous silicon solar cell utilizing a p-i-n junction of a semiconductor have been developed for practical application. However, since the production cost of a silicon solar cell is relatively high and much energy is consumed in the production process, it is necessary to use the solar cell for a long duration in order to compensate the production cost and the consumed energy. Especially, the high production cost interferes with the wide use of a silicon solar cell.
On the other hand, recently, solar cells using CdTe and CuIn(Ga)Se have been studied for practical application as second generation thin film solar cells. Regarding the solar cells using these materials, problems with environmental pollution and resource consumption have been observed.
In addition to those dry type solar cells using a semiconductor junction, there is also suggested a wet type solar cell utilizing a photoelectric chemical reaction caused in the interface of a semiconductor and an electrolytic solution. A metal oxide semiconductor such as titanium oxide, tin oxide, or the like used for the wet solar cell has an advantage of lowering solar-cell manufacturing cost as compared with silicon, gallium arsenide, or the like used for the foregoing dry type solar cells. Above all, titanium oxide is expected to be a future energy conversion material since it is excellent in both photoelectric conversion efficiency in an ultraviolet region and stability. Since a stable semiconductor such as titanium oxide, however, has a wide band gap not less than 3 eV, only ultraviolet rays, which are about 4% of sunrays, can be utilized, and the photoelectric conversion efficiency has been insufficient.
For this reason, a photochemical cell (dye-sensitized wet type solar cell) comprising a photoelectric semiconductor adsorbing dye on the surface has been studied. At the beginning, a single crystal electrode of a semiconductor was used for such a photochemical cell. Examples of such electrode are titanium oxide, zinc oxide, cadmium sulfide, tin oxide, or the like. Since an amount of the coloring agent to be adsorbed on the single crystal electrode lowered photoelectric conversion efficiency and the cost was high, a porous semiconductor electrode was then used. Tubomura et al. (NATURE, 261 (1976) p. 402) reported that the photoelectric conversion efficiency had been improved by adsorbing dye in a semiconductor electrode made of a porous zinc oxide produced by sintering a fine particle. Proposals of employing porous semiconductor electrodes were also made in Japanese Patent Application Laid-Open No. 10-112337 and Japanese Patent Application Laid-Open No. 9-237641.
Graetzel et al. (J. Am. Chem. Soc. 115 (1993) 6382, U.S. Pat. No. 5,350,644) also reported that performance as high as that of a silicon solar cell was achieved by improving dye and a semiconductor electrode. There, a ruthenium type coloring agent was used as dye and an anatase type porous titanium oxide (TiO2) was used as a semiconductor electrode.
FIG. 6 is a schematic cross-sectional view of a photochemical cell using the dye-sensitized semiconductor electrode reported by Graetzel et al. (hereafter referred to as a Graetzel type cell). FIG. 6 shows an outline structure and functions of the cell.
In FIG. 6, 14a and 14b denote a glass substrate, 15a and 15b denote a transparent electrode formed on a glass substrate, and 61 denotes an anatase type porous titanium oxide semiconductor layer composed of fine titanium oxide particles bonded to one another in a porous state. Further, 62 denotes a light absorption layer of dye bonded to the surface of the fine titanium oxide particles and 63 denotes an electron donative electrolytic solution. An electrolytic solution containing iodine ions may be employed as the electron donative electrolytic solution.
A method of manufacturing a Graetzel type cell will be described below.
At first, a layer of an anatase type titanium oxide fine particle is formed on a glass substrate 14a on which a transparent electrode 15a is formed. Various kinds of formation methods are available, and generally, formation of an approximately 10 μm thick semiconductor layer 61 of an anatase type titanium oxide fine particle is carried out by applying a paste containing dispersed anatase type titanium oxide fine particles with 10 to 20 μm particle diameter to a transparent electrode 15a and then firing the paste at 350 to 500° C. Such a method can provide a layer with about 50% porosity and about a 1000 roughness factor (practical surface area/apparent surface area), in which the fine particles are well bonded to one another.
After that, dye is adsorbed in the produced titanium oxide layer 61. Various kinds of substances have been studied for use as dye and generally a Ru complex is utilized. The titanium oxide layer 61 is immersed in a solution containing dye and dried to bind the coloring agent to the surfaces of the titanium oxide fine particles and to form a light absorption layer 62. A substance which does not inhibit adsorption of dye in a titanium oxide layer, is capable of dissolving dye well and is electrochemically inert even if remaining on the surface of the electrode (the transparent electrode and the titanium oxide) is suitable as a solvent to dissolve the coloring agent, and from that point, ethanol and acetonitrile are preferably used.
Further, as an opposed electrode, a glass substrate 14b on which a transparent electrode 15b is formed is made ready and an ultra thin film of platinum or graphite is formed on the surface of the transparent electrode 15b. The ultra thin film works as a catalyst at the time of transporting electric charge to and from an electrolytic solution 63.
After that, while the transparent electrode 15a and 15b being set in the inner sides, the glass substrates 14a and 14b are overlaid as to hold the electrolytic solution 63 between them to give a Graetzel type cell. Acetonitrile, propylene carbonate, or the like, which are electrochemically inert and capable of dissolving a sufficient amount of an electrolytic substance, are preferably used as a solvent for the electrolytic solution 63. As an electrolytic substance, a stable redox pair such as I−/I3−, Br−/Br3− is preferably used. At the time of forming, for example, a pair of I−/I3−, a mixture of iodine ammonium salt and iodine, is used as a solute of the electrolytic solution 63.
Finally, it is preferable to seal the obtained cell with an adhesive to provide durability.
Next, the action principle of the Graetzel type cell will be described below. Light is radiated to the Graetzel type cell from the left side shown in FIG. 6. Subsequently, electrons of the coloring agent constituting the light absorption layer 62 are excited owing to the incident light. The excited electrons are efficiently injected to the titanium oxide layer 61 and transferred to a conduction band of titanium oxide. The coloring agent which loses electrons and falls into an oxidized state is quickly reduced by receiving electrons from iodine ions in the electrolytic solution 63 and is returned to its original state. The electrons injected into the titanium oxide layer 61 are moved owing to a mechanism such as hopping conduction among the titanium oxide fine particles and reach the anode 15a (the left side transparent electrode in FIG. 6). On the other hand, the iodine ions which are in oxidized state (I3−) by supplying electrons to the coloring agent are reduced by receiving electrons from the cathode (the right side transparent electrode in FIG. 6) 15b and turn back to their original state (I−).
As is suggested by such an action principle, in order to efficiently separate the electrons and the holes generated in the coloring agent and move them, the energy level of the electrons of the coloring agent in the excited state has to be higher than that of the conduction band of titanium oxide, and the energy level of the holes of the coloring agent has to be lower than the redox level of iodine ion.
Further improvements on the photoelectric conversion efficiency, the short circuit current, the open circuit voltage, the filter factor, and durability are desirable to promote replacement of a silicon solar cell with such a Graetzel type cell.
However, since the foregoing coloring agent-sensitized semiconductor electrode is a titanium oxide film produced by applying the solution containing dispersed titanium oxide fine particles to the transparent conductive film (the transparent electrode) 15a and sintering at high temperature after drying, the excited electrons tend to be scattered in the interfaces of the transparent electrode, in the titanium oxide fine particles and in the interfaces of titanium oxide fine particles themselves. The internal resistance generated in the interfaces of the transparent electrode, the titanium oxide fine particles and in the interfaces of titanium oxide fine particles themselves, therefore, is increased to result in a decrease in photoelectric conversion efficiency. Moreover, movement of the excited electrons to the redox system or the like in the interfaces of the titanium oxide fine particles themselves also causes decrease of the photoelectric conversion efficiency.
Further, since the foregoing coloring agent-sensitized semiconductor electrode comprises a sintered body of titanium oxide fine particles, problems are caused, such as, adsorption of dye in the titanium oxide fine particles located in the periphery of the transparent electrode takes a long time, and diffusion of ions in the electrolytic solution 63 is slow.